Blender Graph editor Continuos animation
To create a continuous line (rather than between two points) you can add an F-curve Modifier - see https://docs.blender.org/manual/en/dev/editors/graph_editor/fcurves/modifiers.html

The Generator modifier allows you define the relationship between X and Y to generate the required graph. In your case you want polynomial order of 1 so it is of the form y = mx + c - ie, a straight line with the gradient m and cutting through the y-axis at c.